This invention relates to a surge absorber suitable for electronic communication equipment, such as, telephone sets, facsimiles, telephone switch-boards, modems, and the like. More particularly, it relates to a surge absorber capable of protecting electronic devices from the invasion of continuous overvoltages or overcurrents in addition to absorbing surge voltages applied to the electronic device.
In the prior art, the surge absorbing element is connected to a pair of input lines of an electronic device in parallel with the electronic device, and is designed to operate at a higher voltage than the operating voltage of the electronic device. Such a prior art surge absorbing element is a resistor having a high resistance value when the voltage applied thereto is lower than the discharge starting voltage thereof, but its resistance value is equal to as low as several tens of ohms or less when the voltage applied thereto is equal to or higher than the discharge starting voltage thereof. When surge voltages, such as, lightning surges, etc., are instantaneously applied to an electronic device, the surge absorbing element discharges to absorb the surge voltages, and serves to protect the electronic device from the surge voltages. However, when overvoltages or overcurrents due to accidents are continuously applied to the electronic circuit including the electronic device, a certain amount of current continuously flows through the surge absorbing element. This results in the surge absorbing element being heated to high temperatures. The heat radiating from the surge absorbing element can cause the electronic equipment surrounding the surge absorbing element to catch fire. Such accidents include a case where, for example, a power supply line connected to the electronic device comes into contact with a communication line connected to the electronic device.
While it does not usually happen that such accidental overvoltages or overcurrents are continuously applied to the circuit, it has recently become more desirable in many fields to take the maximum safety measures to avoid such accidental problems. As an example, UL (Underwriter's Laboratories Inc.) of the U.S.A. prescribes a safety standard for surge absorbers so that they do not cause fire or electrical shock in communication equipment when continuous overvoltages or overcurrents are applied.
Conventionally, for a surge absorber to conform with such safety standards, a surge absorbing element is known in which a fuse is adhered on and connected to one-side of the input lines of the electronic device. However, heating of the surge absorbing element cannot be prevented if it is subjected continuously with to an alternating-current less than the rated current of the fuse and an overvoltage equal to or more than the operation voltage of the surge absorbing element.
To solve this problem, as shown in FIGS. 4 and 5, the surge absorbing element 14 is connected in series with a fuse wire 20. The fuse wire 20 is fused (melted) from the heat of the surge absorbing element 14.
By using the surge absorber described above, damage to the surge absorbing element and the electronic device can be prevented by fusing when the overvoltages or overcurrents are applied thereon. However, the device is disadvantageous in that it requires many man-hours for connecting the fuse wire, and is not suitable for mass production. In addition, caution must be taken in handling it so that the fuse wire is not cut erroneously by the hands or fingers after the fuse wire is connected.